Over the years there has been a growing interest in the field of miniature sensors. Applications for miniature sensors are wide ranging and include medical implants and embedded sensors in buildings. One specific area that has received little attention is how to supply the required electrical power to such sensors. Conventional power supplies, such as batteries, can be disposed external to such sensors. However, certain applications require the sensors to be completely embedded in the structure with no physical connection to a location outside the structure. Supplying power to such systems is generally difficult. As a result, these sensors typically need their own self-powered power supply.
Batteries are not generally a viable solution for devices such as embedded sensors. Batteries contain a finite amount of energy and have a limited lifetime. Batteries also can contain hazardous chemicals, can be quite bulky and can fail without notice.
Self powered microsystems can convert energy from an existing ambient energy source into a different form of energy, such as electrical energy. Electrical generators based on self powered microsystems can be used to generate low noise electrical power, virtually eliminate cross talk between power lines and signal lines, and can operate efficiently with a relatively simple power delivery and control system.
Some possible ambient energy sources which can be converted into electrical energy include light energy, thermal energy, volume flow energy and mechanical energy. However, mechanical vibrations may be the only feasible ambient source of energy when the former energy sources are typically unavailable, such as in embedded applications where there is no light, no flow, and zero temperature gradient.
The transformation of mechanical vibrations into electrical power is generally accomplished using the piezoelectric effect. The piezoelectric effect results in a voltage being generated across dielectric crystals which are subjected to mechanical stress. For example, micro-mechanical generators (micromachined silicon/piezoelectric device) may be constructed by depositing thin-films of lead zirconate titanate (PZT) onto single crystal silicon. Single crystal silicon possesses excellent mechanical properties for this type of application.
Although self powered microsystems can generate their “own” energy, they generally require an energy storage device as the output generated may vary rapidly with time. The ambient energy may also not be present at all times or some start-up power may be required. Ideally the chosen energy storage device exhibits high power and energy densities with the smallest size possible. Batteries (e.g. Li batteries) and capacitors, including supercapacitors may be used as energy storage devices in conjunction with self powered microsystems.
Vibrational energy harvesting approaches may be categorized in terms of physical size and the transduction approach. Meso-scale energy harvesting approaches have included rotary generators embedded in a boot, a moving coil generator, piezoelectric patches placed in the heel and midsole of a shoe, and a dielectric elastomer with compliant electrodes. For the meso-scale harvesters, the reported power output ranges from 400 μW to over 500 mW for single generator units, but this figure does not take into account variation in sizes and force inputs.
To match a generator and the load impedances for optimal power transfer, a power converter is generally inserted between the generator and the load. Due to their simplicity, pulsewidth-modulated (PWM) converters have been the preferred choice for the matching function. In order to keep the waveforms piecewise linear, however, the inductances in these converters need to be scaled inversely with power level and switching frequency.
Since the power level is typically only in the sub-milliwatt range for vibrational energy harvesting and the switching frequency is generally kept in the sub-kilohertz range, inductances in the 1-10 millihenry range have been reported. The corresponding volume is on the order of a few cubic inches. Such a volume is clearly not compatible with die-based MEMS fabrication processing which requires inductances in the power converters be in the sub-microhenry range to achieve a reasonable die size. Moreover, the power conversion efficiency of disclosed vibrational energy harvesting systems has been low primarily because the switching frequency used has not been optimized to keep the power losses in the power converter substantially less than the amount of power being extracted.